Upgrading light naphtas for increased olefins production

ABSTRACT

The present invention is a process to upgrade light naphthas comprising branched paraffins and their use as a feedstock in a steam cracking unit, said light naphthas consisting essentially of 90 to 100% by weight of hydrocarbons having at least 5 and up to 8 carbon atoms, said process comprising, 
     a) optionally providing an isomerization zone recovered from the gasoline unit of an oil refinery,
 
b) optionally providing a separation zone capable to treat an hydrocarbon stream comprising branched paraffins and normal paraffins to produce a first hydrocarbon stream having a reduced branched paraffins content and an enhanced normal paraffins content and a second hydrocarbon stream having an enhanced branched paraffins content and a reduced normal paraffins content,
 
c) optionally providing a depentanizer, such that at least two of a), b) and c) are present,
 
wherein,
 
the light naphtha is sent to one of a), b) and c),
 
streams are circulating between the various zones a), b) or c),
 
a stream rich in normal paraffins is sent to the steam cracking unit.

FIELD OF THE INVENTION

The present invention relates to the upgrading of light naphthas forincreased olefins production. Light naphthas are becoming widelyavailable as feedstocks for steam crackers due to the difficulties inits valorization as components of the gasoline pool in refineries. Thequality of these naphthas as feeds to steam crackers could be improvedby increasing the amount of normal paraffins at the expense ofiso-paraffins, which would increase the yield of ethylene while reducingyields of fuel gas, C4s and pygas. The isomerization reaction betweennormal and iso-paraffins is essentially limited by thermodynamicequilibrium. This has led to consider whether it would be feasible tooperate the refinery isomerization units driving the reaction in areverse mode, namely transforming iso-paraffins into normal paraffins.

BACKGROUND OF THE INVENTION

GB 2 018 815 A1 describes a process for converting unsaturated C4hydrocarbons into normal butane and more particularly to the conversionof normal butenes and isobutene into normal butane. The normal butanecan thereafter be either recovered, or advantageously, recycled to theethylene process as a premium cracking feedstock to increase the overallyield of ethylene. In a preferred embodiment the feed stream of normalbutenes and isobutene is obtained from unsaturated C4 hydrocarbonsgenerated in the recovery zone of a conventional ethylene productionfacility. The crude unsaturated C4 hydrocarbon by-product streamseparated in the recovery zone of a naphtha steam cracker is usuallydirected to a butadiene recovery facility where high purity1,3-butadiene is separated from the remaining C4 hydrocarbons. Theremaining C4 hydrocarbons are withdrawn from the butadiene recoveryfacility primarily as a mixture known in the art as “butene raffinate.”This mixture is generally comprised of normal butenes and isobutene. Theprocess comprises passing a stream of unsaturated C4 hydrocarbons incontact with hydrogen through a hydrogenation zone to react the hydrogenand the unsaturated C4 hydrocarbons to form normal butane and isobutane.The normal butane and isobutane are discharged from the hydrogenationzone and are introduced into a separation zone to separate the normalbutane from the isobutane. The normal butane is discharged and recoveredfrom the separation zone. The isobutane from the separation zone ispassed into an isomerization zone to convert a portion of the isobutaneinto normal butane to form a stream of normal butane and isobutane.Thereafter, the normal butane and isobutane stream formed in theisomerization zone is withdrawn from the isomerization zone. This streamcan thereafter be directed to the same separation zone which separatesthe normal butane and isobutane introduced from the hydrogenation zoneto recover additional amounts of normal butane.

U.S. Pat. No. 5,019,661 provides a single-stage process for theshape-selective hydroisomerization of a branched olefin of at least 4carbon atoms to produce a less branched paraffin product, said processcomprising contacting said olefin and a hydrogen-containing gas with azeolite or zeolite-like catalyst containing at least one metal of GroupVIII and in which a major portion of said at least one of these metalsis supported within the molecular channels and cavities of the saidcatalyst, said process being conducted under conditions such thathydroisomerization predominates over both simple hydrogenation andcracking. It is an object of the present invention to provide a processwhereby olefinic feedstocks may be hydroisomerised to paraffinicfeedstocks and specific catalysts therefor. By way of example,2-methylpent-1-ene or 3,3-dimethylbut-1-ene may be hydroisomerised to aproduct containing large amounts of n-hexane. There is no mention of theuse of said process (i) to enhance the normal paraffin content of aparaffin fraction and further (ii) use said enhanced normal paraffinfraction as a feedstock in a steam cracker to make olefins.

US 2005 101814 relates to a process for the production of light olefinsfrom a naphtha feed stream. The naphtha is sent to an adsorptiveseparation unit which produces a first process stream comprisingprimarily n-paraffins, and a second process stream comprising non-normalhydrocarbons. The second process stream is processed through a ringopening reactor that hydrogenates and converts the aromatics andnaphthenes to paraffins. The paraffins from the adsorptive separationunit and from the hydrogenation ring opening reactor are then passedthrough a steam cracking unit to produce light olefins. This processincreases the yield of light olefins from a naphtha feedstream. Theprocess may optionally include the passing of a py-gas stream generatedin the steam cracking unit to the ring opening reactor to furtherincrease the light olefin production. In an alternate process theparaffins from the hydrogenation ring opening reactor are then passedthrough an isomerization unit for the conversion of a portion of theiso-paraffins to normal paraffins, and the resulting mixture is recycledto the adsorption unit. The isomerization unit increases light olefinproduction by increasing the amount of normal paraffins recovered fromthe naphtha feed stream. This process concerns only the treatment of theconventional naphtha feedstock to be used in a steam cracker.

The present invention relates to the use of an existing isomerizationzone of an oil refinery to increase the normal paraffins content of alight naphtha which is further cracked in a steam cracking unit toproduce olefins.

BRIEF SUMMARY OF THE INVENTION

In a first embodiment the present invention is a process to upgradelight naphthas comprising branched paraffins and their use as afeedstock in a steam cracking unit, said light naphthas consistingessentially of 90 to 100% (advantageously 95 to 100%) by weight ofhydrocarbons having at least 5 and up to 8 carbon atoms, said processcomprising,

-   a) providing an isomerization zone recovered from the gasoline unit    of an oil refinery,-   b) providing a separation zone capable to treat an hydrocarbon    stream comprising branched paraffins and normal paraffins to produce    a first hydrocarbon stream having a reduced branched paraffins    content and an enhanced normal paraffins content and a second    hydrocarbon stream having an enhanced branched paraffins content and    a reduced normal paraffins content,-   c) optionally providing a depentanizer,-   d) sending the light naphtha to the isomerization zone and operating    said zone at conditions effective to produce a light naphtha having    a reduced branched paraffins content and an enhanced normal    paraffins content,-   e) sending the withdrawn light naphta from step d) to the separation    zone to recover a first and a second hydrocarbon streams,-   f) sending the first hydrocarbon stream recovered from step e) to    the steam cracking unit,-   g) recycling at least a part of the second hydrocarbon stream    recovered from step e) at the inlet of the isomerization zone,-   h) optionally, before the recycling of step g), sending the second    hydrocarbon stream recovered at step e) to a depentanizer to recover    a stream comprising essentially pentane and a stream having a    reduced pentane content and sending at least a part of said stream    comprising essentially pentane at the inlet of the isomerization    zone.

In a second embodiment the present invention is a process to upgradelight naphthas comprising branched paraffins and their use as afeedstock in a steam cracking unit, said light naphthas consistingessentially of 90 to 100% (advantageously 95 to 100%) by weight ofhydrocarbons having at least 5 and up to 8 carbon atoms, said processcomprising,

-   a) providing an isomerization zone recovered from the gasoline unit    of an oil refinery,-   b) providing a separation zone capable to treat an hydrocarbon    stream comprising branched paraffins and normal paraffins to produce    a first hydrocarbon stream having a reduced branched paraffins    content and an enhanced normal paraffins content and a second    hydrocarbon stream having an enhanced branched paraffins content and    a reduced normal paraffins content,-   c) sending the light naphtha to the separation zone to recover a    first and a second hydrocarbon streams,-   d) sending the second hydrocarbon stream recovered from step c) to    the isomerization zone and operating said zone at conditions    effective to produce a light naphtha having a reduced branched    paraffins content and an enhanced normal paraffins content,-   e) mixing the outlet stream of step d) with the first hydrocarbon    stream recovered from step c) and sending said mixed stream to the    steam cracking unit.

In a third embodiment the present invention is a process to upgradelight naphthas comprising branched paraffins and their use as afeedstock in a steam cracking unit, said light naphthas consistingessentially of 90 to 100% (advantageously 95 to 100%) by weight ofhydrocarbons having at least 5 and up to 8 carbon atoms, said processcomprising,

-   a) providing an isomerization zone recovered from the gasoline unit    of an oil refinery,-   b) providing a separation zone capable to treat an hydrocarbon    stream comprising branched paraffins and normal paraffins to produce    a first hydrocarbon stream having a reduced branched paraffins    content and an enhanced normal paraffins content and a second    hydrocarbon stream having an enhanced branched paraffins content and    a reduced normal paraffins content,-   c) optionally providing a depentanizer,-   d) sending the light naphtha to the separation zone to recover a    first and a second hydrocarbon streams,-   e) sending the first hydrocarbon stream recovered from step d) to    the steam cracking unit,-   f) sending at least a part of the second hydrocarbon stream    recovered from step d) to the isomerization zone and operating said    zone at conditions effective to produce a light naphtha having a    reduced branched paraffins content and an enhanced normal paraffins    content,-   g) optionally, before sending the second hydrocarbon recovered from    step d) to the isomerization zone, sending said hydrocarbon stream    to a depentanizer to recover a stream comprising essentially    iso-pentane and a stream having a reduced pentane content and    sending said stream comprising essentially iso-pentane to the    isomerization zone,-   i) recycling the outlet stream from step f) to the inlet of the    separation zone.

In a fourth embodiment the present invention is a process to upgradelight naphthas comprising branched paraffins and their use as afeedstock in a steam cracking unit, said light naphthas consistingessentially of 90 to 100% (advantageously 95 to 100%) by weight ofhydrocarbons having at least 5 and up to 8 carbon atoms, said processcomprising,

-   a) providing an isomerization zone recovered from the gasoline unit    of an oil refinery,-   b) providing a deisopentanizer,-   c) sending the light naphtha to a deisopentanizer to recover a    stream comprising essentially isopentane and a stream having a    reduced isopentane content,-   d) sending the stream having a reduced isopentane content recovered    from step c) to the steam cracking unit,-   e) sending the stream comprising essentially isopentane recovered    from step c) to the isomerization zone and operating said zone at    conditions effective to produce a light naphtha having a reduced    branched paraffins content and an enhanced normal paraffins content,-   f) recycling the outlet of step e) to the inlet of the    deisopentanizer.

In the present invention Cn+ means an hydrocarbon having n carbon atomsor more and Cn means an hydrocarbon having n carbon atoms.

The fourth embodiment is of particular interest when the light naphthato be upgraded is a C5 cut comprising, the total being 100 w %,

-   0 to 10% of C7+, advantageously 0 to 10% of C6+, more advantageously    0 to 10% of C6,-   0 to 10% of C4,-   80 to 100% of C5, advantageously said C5 is essentially a mixture of    iC5 and nC5. Advantageously said C5 mixture comprises less than 5 w    % of NaftC5 (C5 napthenics), preferably less than 3%.

The fourth embodiment is of particular interest when the light naphthato be upgraded is a C5 cut comprising, the total being 100 w %,

-   0 to 5% of C6+, advantageously 0 to 5% of C6,-   0 to 5% of C4,-   90 to 100% of C5, advantageously said C5 is essentially a mixture of    iC5 and nC5. Advantageously said C5 mixture comprises less than 5 w    % of NaftC5 (C5 napthenics), preferably less than 3%.

The fourth embodiment is of particular interest when the light naphthato be upgraded is a C5 cut comprising, the total being 100 w %,

-   0 to 3% of C6+, advantageously 0 to 3% of C6,-   0 to 3% of C4,-   94 to 100% of C5, advantageously said C5 is essentially a mixture of    iC5 and nC5. Advantageously said C5 mixture comprises less than 5 w    % of NaftC5 (C5 napthenics), preferably less than 3%.

Advantageously in the 4 above embodiments the isomerization zoneoperates in the presence of hydrogen.

DETAILED DESCRIPTION OF THE INVENTION

As regards the light naphtha to be upgraded, one can cite by way ofexample a C5 and a C5/C6 naphtha.

The light naphtha to be upgraded can be a C5 cut comprising, the totalbeing 100 w %, 100 to 95% of a mixture of pentane and isopentane and 0to 5% of cyclopentane.

-   The light naphtha to be upgraded can be a C5/C6 cut comprising, the    total being 100 w %,-   0 to 10% of C7+, advantageously 0 to 10% of C7,-   0 to 10% of C4,-   80 to 100% of a mixture of normal and branched C5 and C6.

In an embodiment the light naphtha to be upgraded can be a C5/C6 cutcomprising, the total being 100 w %,

-   0 to 10% of C7+, advantageously 0 to 10% of C7,-   0 to 10% of C4,-   20 to 60% of a mixture of normal and branched C5,-   20 to 60% of a mixture of normal and branched C6.    The light naphtha to be upgraded can be a C5/C6 cut comprising, the    total being 100 w %,-   0 to 5% of C7+, advantageously 0 to 5% of C7,-   0 to 5% of C4,-   90 to 100% of a mixture of normal and branched C5 and C6.    The light naphtha to be upgraded can be a C5/C6 cut comprising, the    total being 100 w %,-   0 to 2% of C7+, advantageously 0 to 2% of C7,-   0 to 5% of C4,-   93 to 100% of a mixture of normal and branched C5 and C6.    In the above compositions of light naphta to be upgraded    advantageously the proportion of NaftC5+NaftC6 is less than 7 w %,    preferably less than 5%.    In the above compositions of light naphta to be upgraded    advantageously the proportion of Aromatics C6 is less than 5 w %,    preferably less than 2% more preferably less than 1.5%.

As regards the isomerization zone, FIG. 1 shows the equilibrium betweenthe C5 and C6 normal and branched paraffins as function of the reactiontemperature. It can be seen that the formation of iso-paraffins is morefavorable at higher temperatures although in the considered range it isalways found a larger proportion of iso-paraffins. Equilibrium is morefavorable for the production of n-C5 than n-C6. The figure alsoindicates that at higher temperatures other catalytic concepts could beused. It appears clearly from FIG. 1 that high reaction temperaturesshould be preferred for driving the reverse reaction towards n-paraffinsand at those conditions a zeolite catalyst could be more advisable.However since existing isomerization units in refineries were designedmainly at low temperature, this case could be also interesting as aminimum investing option.

Any catalyst known in the art to be suitable for the isomerization ofparaffin-rich hydrocarbon streams may be used as an isomerizationcatalyst in the isomerization zone. One suitable isomerization catalystcomprises a platinum-group metal, hydrogen-form crystallinealuminosilicate zeolite and a refractory inorganic oxide, and thecomposition preferably has a surface area of at least 580 m2 /g. Thepreferred noble metal is platinum, which is present in an amount of fromabout 0.01 to 5 mass % of the composition, and optimally from about 0.15to 0.5 mass %. Catalytically effective amounts of one or more promotermetals preferably selected from Groups VIB(6), VIII(8-10), IB(11),IIB(12), IVA(14), rhenium, iron, cobalt, nickel, gallium and indium alsomay be present. The crystalline aluminosilicate zeolite may be syntheticor naturally occurring, and preferably is selected from the groupconsisting of FAU, LTL, MAZ and MOR with mordenite having asilica-to-alumina ratio of from 16:1 to 60:1 being especially preferred.The zeolite generally comprises from about 50 to 99.5 mass % of thecomposition, with the balance being the refractory inorganic oxide.Alumina, and preferably one or more of gamma-alumina and eta-alumina, isthe preferred inorganic oxide. Further details of the composition aredisclosed in U.S. Pat. No. 4,735,929, incorporated herein in itsentirety by reference thereto. A preferred isomerization catalystcomposition comprises one or more platinum-group metals, a halogen, andan inorganic-oxide binder. Preferably the catalyst contains aFriedel-Crafts metal halide, with aluminum chloride being especiallypreferred. The optimal platinum-group metal is platinum which is presentin an amount of from about 0.1 to 5 mass %. The inorganic oxidepreferably comprises alumina, with one or more of gamma-alumina andeta-alumina providing best results. Optimally, the carrier material isin the form of a calcined cylindrical extrudate. The inlet stream of theisomerization zone may also contain an organic polyhalo component, withcarbon tetrachloride being preferred, and the total chloride content isfrom about 2 to 15 mass %. An organic-chloride promoter, preferablycarbon tetrachloride, is added during operation to maintain aconcentration of 30 to 300 mass ppm of promoter in the combined feed.Other details and alternatives of preparation steps and operation of thepreferred isomerization catalyst are as disclosed in U.S. Pat. Nos.2,999,074 and 3,031,419 which are incorporated herein by reference.

Hydrogen is advantageously mixed with the inlet stream of theisomerization zone to provide a mole ratio of hydrogen to hydrocarbonfeed of about 0.01 to 5. The hydrogen may be supplied totally fromoutside the process or supplemented by hydrogen recycled to the feedafter separation from reactor effluent. Light hydrocarbons and smallamounts of inserts such as nitrogen and argon may be present in thehydrogen. Water should be removed from hydrogen supplied from outsidethe process, preferably by an adsorption system as is known in the art.In a preferred embodiment the hydrogen to hydrocarbon mol ratio in thereactor effluent is equal to or less than 0.05, generally obviating theneed to recycle hydrogen from the reactor effluent to the feed.

Water and sulfur are catalyst poisons especially for the chloridedplatinum-alumina catalyst composition described herein below. Water canact to permanently deactivate the catalyst by removing high-activitychloride from the catalyst, and sulfur temporarily deactivates thecatalyst by platinum poisoning. A hydrotreating or hydrorefining orhydrodesulfurization step usually reduces water-generating oxygenates tothe preferred required 0.1 ppm or less and sulfur to 0.5 ppm or less.Other means such as adsorption systems for the removal of sulfur andwater from hydrocarbon streams are well known to those skilled in theart.

Isomerization conditions in the isomerization zone include reactortemperatures usually ranging from about 50 to 350° C. Higher reactiontemperatures are generally preferred in order to favour equilibriummixtures having the highest concentration of normal alkanes.Temperatures in the range of about 150 to about 250° C. are preferred inthe present invention. Reactor operating pressures generally range fromabout 100 kPa to 10 MPa absolute, preferably between about 0.5 and 4MPa. Liquid hourly space velocities range from about 0.2 to about 15volumes of isomerizable hydrocarbon feed per hour per volume ofcatalyst, with a range of about 0.5 to 5 hr-1 being preferred.

Contacting within the isomerization zone may be effected using thecatalyst in a fixed-bed system, a moving-bed system, a fluidized-bedsystem, or in a batch-type operation. A fixed-bed system is preferred.The reactants may be contacted with the bed of catalyst particles ineither upward, downward, or radial-flow fashion. The reactants may be inthe liquid phase, a mixed liquid-vapour phase, or a vapour phase whencontacted with the catalyst particles, with excellent results beingobtained by application of the present invention to a primarilyliquid-phase operation. The isomerization zone may be in a singlereactor or in two or more separate reactors with suitable means therebetween to insure that the desired isomerization temperature ismaintained at the entrance to each zone. Two or more reactors insequence are preferred to enable improved isomerization through controlof individual reactor temperatures and for partial catalyst replacementwithout a process shutdown.

The isomerization of light naphthas is typically performed in a fixedbed reactor operated at temperatures around about 200° C. to about 250°C. in order to favour the formation of normal compounds. Under theseconditions it is usually preferred to use a catalyst containing a noblemetal supported on a chlorated-alumina.

Cooling or heating of the stream at the inlet of the isomerization zonemay be appropriate for temperature flexibility or for the start-up ofthe process.

Separation of normal from iso paraffins can be done using an adsorptionprocess that separates both types of species by using a shape selectivezeolite. These separation processes could be also useful to enhance thereverse reaction by recycling iso-paraffins instead of n-paraffins as inthe conventional process.

As regards the deisopentanizer and the depentanizer, this is known inthe art. The depentanizer is a conventional fractionation to separatethe C5 from the C6 and above. The deisopentanizer is known as a superfractionation and separates the iC5 from the C5 cut.

As regards the separation zone, This is known in the art. The adsorptionseparation unit may be of any suitable type that is appropriate for thespecific situation of the process. The adsorption unit is comprised of abed of adsorbent comprised of a molecular sieve or other appropriateadsorbent for adsorbing hydrocarbons. Examples of suitable adsorptionseparation units include, but are not limited to, swing bed or simulatedmoving bed adsorption units. The inlet stream is separated in theadsorption unit by the selective adsorption and retention of normalparaffins in the adsorption bed. The adsorption separation processundergoes an adsorption step, wherein selected components of the inletstream are adsorbed onto the adsorbent, and followed by a desorptionstep wherein the selected components are desorbed from the adsorbent. Inthis case, the selected components are the normal paraffins. The normalparaffins remain on the adsorbent until a desorbent is passed throughthe adsorption unit.

During the adsorption step, the normal paraffins are separated from theinlet stream by adsorption onto the adsorbent. The remaining componentsof the inlet stream are non-normal (branched) hydrocarbons and passthrough the adsorption bed unaffected. The non-normal hydrocarbons passout of the adsorption unit as a raffinate stream containing a portion ofthe desorbent (remaining in the adsorbent bed further to the desorptionstep). Said raffinate is fractionated to separate the desorbent andrecover the second hydrocarbon stream having an enhanced branchedparaffins content and a reduced normal paraffins content.

During the desorption step, a desorbent is delivered to the adsorptionunit and passes through the adsorbent bed. The desorbent has propertieswhich enable it to displace the heavier normal paraffins from theadsorbent, resulting in the formation of an extract stream. The extractstream comprises normal hydrocarbons and a portion of the desorbentmaterial. The extract stream is fractionated to recycle the desorbentand recover the second hydrocarbon stream having an enhanced branchedparaffins content and a reduced normal paraffins content.

One can cite the Molex® process of UOP and process described in U.S.Pat. No. 3,392,113 and U.S. Pat. No. 3,455,815.

FIG. 2 depicts a process according to the first embodiment of theinvention. The light naphtha is sent via line 1 and 2 to theisomerization zone to produce a light naphtha having a reduced branchedparaffins content and an enhanced normal paraffins content. The effluentis withdrawn via line 3 and sent to the separation zone to recover afirst hydrocarbon stream 4 having a reduced branched paraffins contentand an enhanced normal paraffins content and a second hydrocarbon stream5 having an enhanced branched paraffins content and a reduced normalparaffins content. The first hydrocarbon stream 4 is sent to the steamcracking unit (not shown). A part of the second hydrocarbon stream 5 isrecycled via line 7 to the isomerization zone and the other part ispurge via line 6.

FIG. 2-a derives from FIG. 2, a depentanizer is inserted after theseparation zone to recycle the pentane at the isomerization zone vialine 7 and purge the C6.

FIG. 3 depicts a process according to the second embodiment of theinvention. The light naphtha is sent via line 1 to the separation zoneto recover a first hydrocarbon stream 4 having a reduced branchedparaffins content and an enhanced normal paraffins content and a secondhydrocarbon stream 3 having an enhanced branched paraffins content and areduced normal paraffins content. The second hydrocarbon stream 3 issent to the isomerization zone operating at conditions effective toproduce a light naphtha 5 having a reduced branched paraffins contentand an enhanced normal paraffins content. The outlet stream 5 is mixedwith the first hydrocarbon stream 4 and said mixed stream 7 is sent tothe steam cracking unit (not shown).

FIG. 4 depicts a process according to the third embodiment of theinvention. The light naphtha is sent via lines 1 and 2 to the separationzone to recover a first hydrocarbon stream 4 having a reduced branchedparaffins content and an enhanced normal paraffins content and a secondhydrocarbon stream 3 having an enhanced branched paraffins content and areduced normal paraffins content. The first hydrocarbon stream 4 is sentto the steam cracking unit (not shown). The second hydrocarbon stream 3is sent to the isomerization zone via line 6 after a purge 5. Theisomerization zone is operated at conditions effective to produce alight naphtha having a reduced branched paraffins content and anenhanced normal paraffins content 7 recycled at the inlet of theseparation zone.

FIG. 4-a derives from FIG. 4 by insertion of a depentanizer. Hydrocarbonstream 3 is sent to a depentanizer to recover a stream 6 comprisingessentially pentane and a stream 5 having a reduced pentane content. Thestream 6 comprising essentially isopentane is sent to the isomerizationzone.

FIG. 5 depicts a process according to the fourth embodiment of theinvention. The light naphtha is sent via line 1 and 2 to adeisopentanizer to recover a stream 5 comprising essentially isopentaneand a stream 4 having a reduced isopentane content. The stream 4 havinga reduced isopentane content is sent to the steam cracking unit (notshown). The stream 5 comprising essentially isopentane is sent to theisomerization zone operated at conditions effective to produce a lightnaphtha 6 having a reduced branched paraffins content and an enhancednormal paraffins content recycled to the inlet of the deisopentanizer.

EXAMPLES Example 1

This ex is made according to FIG. 2. This is a simulation, theisomerization temperature is 300° C., the recovery of normal paraffinsis 99%, the recovery of others is 10% and the purge 20%. in thefollowing table “str 1” corresponds to line 1 on the fig.

Example 2

This ex is made according to FIG. 2-a. This is a simulation, theisomerization temperature is 300° C., the recovery of normal paraffinsis 99%, the recovery of others is 10%.

Example 3

This ex is made according to FIG. 3. This is a simulation, theisomerization temperature is 300° C., the recovery of normal paraffinsis 99%, the recovery of others is 10%. In the following table “str 1”corresponds to line 1 on the fig.

Example 4

This ex is made according to FIG. 3. This is a simulation, theisomerization temperature is 150° C., the recovery of normal paraffinsis 99%, the recovery of others is 10%. In the following table “str 1”corresponds to line 1 on the fig.

Example 5

This ex is made according to FIG. 4. This is a simulation, theisomerization temperature is 250° C., the recovery of normal paraffinsis 99%, the recovery of others is 10% and the purge 37%. In thefollowing table “str 1” corresponds to line 1 on the fig.

Example 6

This ex is made according to FIG. 4-a. This is a simulation, theisomerization temperature is 250° C., the recovery of normal paraffinsis 99%, the recovery of others is 10%. In the following table “str 1”corresponds to line 1 on the fig.

Example 7

This ex is made according to FIG. 5. This is a simulation, theisomerization temperature is 250° C.; In the following table “str 1”corresponds to line 1 on the fig.

Example 8

This ex is made according to FIG. 5. This is a simulation, theisomerization temperature is 150° C. In the following table “str 1”corresponds to line 1 on the fig.

Example 9 Working Conditions

The following working conditions were used:

-   20 g of catalyst loaded without diluents.-   Pressure=30 bar-   The reactor adiabatic and up flow.-   A model charge feed was used: 90% iC5+10% nC5+300 ppm C₂Cl₄ bought    from Air Product®-   H2/Hydrocarbon=0.5 mol/mol (about 90 Nl/l) constant over the test-   VVH=1 and 2 h⁻¹-   T=140° C., 150° C. then 160° C.-   Catalyst: chlorinated alumina (ATIS-2L from Albemarle)-   The dew point was maintained between −47° C. and −56° C. to avoid    catalyst deactivation

The quantity of iC5 converted into nC5 were calculated from the onlineanalyses. The feed was also analyzed to determine eventual presence ofsulfur. No sulfur compounds were detected.

Conversion is calculated with the following formula:

${conversion} = \frac{{iC}\; 5{converted}}{{iC}\; 5{initial}}$

Results of the Test

From the test performed, the following results were obtained:

Maximum of Conversion Obtained in Comparison with the ThermodynamicEquilibrium

Ratio iC5/nC5 at Maximum Conversion Temperature the thermodynamicconversion measured (° C.) equilibrium (thermodynamic) (VVH in h⁻¹) 14077/23 14%  7% (1 h⁻¹) 150 76/24 15% 13% (2 h⁻¹) 160 75/25 16% 14% (2h⁻¹)

Even at an elevated VVH, conversions close to the thermodynamicequilibrium were obtained.

1. Process to upgrade light naphthas comprising branched paraffins andtheir use as a feedstock in a steam cracking unit, said light naphthasconsisting essentially of 90 to 100% by weight of hydrocarbons having atleast 5 and up to 8 carbon atoms, said process comprising, a) providingan isomerization zone recovered from the gasoline unit of an oilrefinery, b) providing a separation zone capable to treat an hydrocarbonstream comprising branched paraffins and normal paraffins to produce afirst hydrocarbon stream having a reduced branched paraffins content andan enhanced normal paraffins content and a second hydrocarbon streamhaving an enhanced branched paraffins content and a reduced normalparaffins content, e) optionally providing a depentanizer, d) sendingthe light naphtha to the isomerization zone and operating said zone atconditions effective to produce a light naphtha having a reducedbranched paraffins content and an enhanced normal paraffins content, e)sending the withdrawn light naphtha from step d) to the separation zoneto recover a first and a second hydrocarbon streams, f) sending thefirst hydrocarbon stream recovered from step e) to the steam crackingunit, g) recycling at least a part of the second hydrocarbon streamrecovered from step e) at the inlet of the isomerization zone, h)optionally, before the recycling of step g), sending the secondhydrocarbon stream recovered at step e) to a depentanizer to recover astream comprising essentially pentane and a stream having a reducedpentane content and sending at least a part of said stream comprisingessentially pentane at the inlet of the isomerization zone.
 2. Processto upgrade light naphthas comprising branched paraffins and their use asa feedstock in a steam cracking unit, said light naphthas consistingessentially of 90 to 100% by weight of hydrocarbons having at least 5and up to 8 carbon atoms, said process comprising, a) providing anisomerization zone recovered from the gasoline unit of an oil refinery,b) providing a separation zone capable to treat an hydrocarbon streamcomprising branched paraffins and normal paraffins to produce a firsthydrocarbon stream having a reduced branched paraffins content and anenhanced normal paraffins content and a second hydrocarbon stream havingan enhanced branched paraffins content and a reduced normal paraffinscontent, c) sending the light naphtha to the separation zone to recovera first and a second hydrocarbon streams, d) sending the secondhydrocarbon stream recovered from step c) to the isomerization zone andoperating said zone at conditions effective to produce a light naphthahaving a reduced branched paraffins content and an enhanced normalparaffins content, e) mixing the outlet stream of step d) with the firsthydrocarbon stream recovered from step c) and sending said mixed streamto the steam cracking unit.
 3. Process to upgrade light naphthascomprising branched paraffins and their use as a feedstock in a steamcracking unit, said light naphthas consisting essentially of 90 to 100%by weight of hydrocarbons having at least 5 and up to 8 carbon atoms,said process comprising, a) providing an isomerization zone recoveredfrom the gasoline unit of an oil refinery, b) providing a separationzone capable to treat an hydrocarbon stream comprising branchedparaffins and normal paraffins to produce a first hydrocarbon streamhaving a reduced branched paraffins content and an enhanced normalparaffins content and a second hydrocarbon stream having an enhancedbranched paraffins content and a reduced normal paraffins content, c)optionally providing a depentanizer, d) sending the light naphtha to theseparation zone to recover a first and a second hydrocarbon streams, e)sending the first hydrocarbon stream recovered from step d) to the steamcracking unit, f) sending at least a part of the second hydrocarbonstream recovered from step d) to the isomerization zone and operatingsaid zone at conditions effective to produce a light naphtha having areduced branched paraffins content and an enhanced normal paraffinscontent, g) optionally, before sending the second hydrocarbon recoveredfrom step h) to the isomerization zone, sending said hydrocarbon streamto a depentanizer to recover a stream comprising essentially pentane anda stream having a reduced pentane content and sending said streamcomprising essentially isopentane to the isomerization zone, i)recycling the outlet stream from step f) to the inlet of the separationzone.
 4. Process to upgrade light naphthas comprising branched paraffinsand their use as a feedstock in a steam cracking unit, said lightnaphthas consisting essentially of 90 to 100% by weight of hydrocarbonshaving at least 5 and up to 8 carbon atoms, said process comprising, a)providing an isomerization zone recovered from the gasoline unit of anoil refinery, b) providing a deisopentanizer, c) sending the lightnaphtha to a deisopentanizer to recover a stream comprising essentiallyisopentane and a stream having a reduced isopentane content, d) sendingthe stream having a reduced isopentane content recovered from step c) tothe steam cracking unit, e) sending the stream comprising essentiallyisopentane recovered from step c) to the isomerization zone andoperating said zone at conditions effective to produce a light naphthahaving a reduced branched paraffins content and an enhanced normalparaffins content, f) recycling the outlet of step e) to the inlet ofthe deisopentanizer.
 5. Process according to claim 1 wherein theisomerization zone operates under the presence of hydrogen.
 6. Processaccording to claim 1 wherein the light naphtha to be upgraded is a C5cut comprising, the total being 100 w %, 100 to 95% of a mixture ofpentane and isopentane and 0 to 5% of cyclopentane.
 7. Process accordingto claim 1 wherein the light naphtha to be upgraded is a C5/C6 cutcomprising, the total being 100 w %, 0 to 10% of C7+, 0 to 10% of C4, 80to 100% of a mixture of normal and branched C5 and C6.
 8. Processaccording to claim 7 wherein the light naphtha to be upgraded is a C5/C6cut comprising, the total being 100 w %, 0 to 5% of C7+, 0 to 5% of C4,90 to 100% of a mixture of normal and branched C5 and C6.
 9. Processaccording to claim 8 wherein the light naphtha to be upgraded is a C5/C6cut comprising, the total being 100 w %, 0 to 2% of C7+, 0 to 5% of C4,93 to 100% of a mixture of normal and branched C5 and C6.
 10. Processaccording to claim 7 wherein in the light naphtha to be upgraded the C7+is a C7.
 11. Process according to claim 7 wherein in the light naphta tobe upgraded the proportion of NaftC5+NaftC6 is less than 7 w %,preferably less than 5%.
 12. Process according to claim 7 wherein in thelight naphta to be upgraded the proportion of Aromatics C6 is less than5 w %.
 13. Process according to claim 12 wherein in the light naphta tobe upgraded the proportion of Aromatics C6 is less than 2 w %. 14.Process according to claim 13 wherein in the light naphta to be upgradedthe proportion of Aromatics C6 is less than 1.5 w %.
 15. Processaccording to claim 4 wherein the light naphtha to be upgraded is a C5cut comprising, the total being 100 w %, 0 to 10 % of C7+, 0 to 10% ofC4, 80 to 100% of C5, said C5 is essentially a mixture of iC5 and nC5.16. Process according to claim 15 wherein in the light naphta to beupgraded the C7+ is a C6+.
 17. Process according to claim 15 wherein thelight naphtha to be upgraded is a C5 cut comprising, the total being 100w %, 0 to 5% of C6+, 0 to 5% of C4, 90 to 100% of C5, said C5 isessentially a mixture of iC5 and nC5.
 18. Process according to claim 17wherein the light naphtha to be upgraded is a C5 cut comprising, thetotal being 100 w %, 0 to 3% of C6+, 0 to 3% of C4, 94 to 100% of C5,said C5 is essentially a mixture of iC5 and nC5.
 19. Process accordingto claim 16 wherein in the light naphta to be upgraded the C6+ is a C6.20. Process according to claim 15 wherein in the light naphta to beupgraded the C5 mixture comprises less than 5 w % of NaftC5 (C5napthenics).
 21. Process according to claim 20 wherein in the lightnaphta to be upgraded the C5 mixture comprises less than 3 w % of NaftC5(C5 napthenics)